1. Field of the Invention
The present invention relates in general to relays and in particular to a current-driven relay having a low profile for use in routing signals on closely spaced circuits or circuit boards of an integrated circuit tester.
2. Description of Related Art
FIG. 1 is a block diagram of a portion of a typical prior art integrated circuit (IC) tester 10 including a set of channels 12, one for each of several terminals of an IC device under test (DUT) 14. Each channel 12 includes a channel control and data acquisition circuit 16, a comparator 18 and a tristate driver 20. A relay 24 links an input of comparator 18 and an output of driver 20 to a DUT terminal 26. Another relay 25 connects a parametric measurement unit (PMU) 28 within channel 12 to DUT terminal 26. A host computer 30 communicates with the channel circuits 16 of each channel 12 via a parallel bus 32.
Tester 10 can carry out both digital logic and parametric tests on DUT 14. Before starting a digital logic test, the control and data acquisition circuit 16 of each channel 12 closes relay 24 and opens relay 25 to connect comparator 18 and driver 20 to DUT terminal 26 and to disconnect PMU 28 from terminal 26. Thereafter, during the digital logic test, the channel control signal may turn on driver 20 and signal it to send a logic test pattern to DUT terminal 26 when the DUT terminal 26 is acting as a DUT input. When terminal 26 is a DUT output, circuit 16 turns off driver 20 and supplies an "expect" bit sequence to an input of comparator 18. Comparator 18 produces an output FAIL signal indicating whether successive states of the DUT output signal matches successive bits of the expect bit sequence. Circuit 16 either stores the FAIL data acquired during the test for later access by host computer 30 or immediately notifies host computer 30 when comparator 18 asserts the FAIL signal.
PMU 28 includes circuits for measuring analog characteristics of the DUT 14 at terminal 26 such as, for example, the DUT's quiescent current. Before starting a parametric test, the channel control circuit 16 opens relay 24 and closes relay 25 to connect the channel's PMU 28 to DUT terminal 26 and to disconnect comparator 18 and driver 20 from terminal 26. Host computer 30 then programs PMU 28 to carry out the parametric test and obtains test results from the PMU.
Relays 24 and 25 are normally preferred over solid state switches for routing signals between DUT 14, PMU 28, driver 20 and comparator 18 because a relay has a very low loss that does not substantially influence test results. We would like to position comparator 18, driver 20, relays 24 and 25, and circuit 16 as close as possible to DUT terminal 26 to minimize the signal path lengths between terminal 26, comparator 18 and driver 20. When the signal paths are too long, the signal delays they cause can make it difficult or impossible to provide the signal timing needed to properly test DUT 14, particularly when the DUT operates at a high speed. Thus to minimize signal path distances we want to use relays 24 and 25 that are as short as possible and which can be reached via short signal paths.
In some prior art testers, one or more channels 12 are implemented on each of a set of printed circuit boards ("pin cards") that are mounted in a cylindrical chassis to form a test head. FIG. 2 illustrates a simplified plan view of a typical test head 34. FIG. 3 is a partial sectional elevation view of the test head 34 of FIG. 2. FIGS. 4 and 5 are expanded front and side elevation views of a lower portion of one of a set of pin cards 36 mounted within test head 34. Pin cards 36 are radially distributed about a central axis 38 of test head 34 and positioned above an integrated circuit device under test (DUT) 14 mounted on a printed circuit board, "load board" 42. A set of pogo pins 44 provide signal paths between relays 24, 25 mounted on pin cards 36 and contact points on the surface of load board 42. Microstrip traces on load board 42 connect the contact points to terminals of DUT 14.
Relays 24, 25 are mounted near the lower edges of each pin card 36 as close as possible to central axis 38 to minimize the signal path distance to DUT 14. However from FIG. 2 we can see that the space between pin cards 36 is relatively limited near axis 38. Thus in order to position relays 24, 25 close to axis 38 we want to use relays that are relatively thin.
FIG. 6 is a simplified sectional elevation view of a typical relay 40. Relay includes a glass tube 42 containing a pair of conductive reeds 44, 45 that serve as the relay's contacts 47. A wire 46 wraps many turns around tube 42 to form a coil 48. Reeds 44, 45 are normally spaced apart, but when a voltage is applied across opposite leads 50, 52 of coil 48, magnetic flux produced by the coil causes reeds 44, 45 to contact one another so that a current may flow through the relay contacts 47. A conductive sheath 43 partially surrounds tube 42 to provide a ground surface. The spacing between reeds 44, 45 and shield 43 influences the characteristic impedance of the transmission line formed by reeds 44 and 45 when they are in contact.
The magnetic force produced by coil 48 on reeds 44, 45 is proportional to the product of the magnitude of the current passing through coil 48 and the number of turns of coil about tube 42. A large number of coil turns is provided to minimize the amount of current needed to operate relay 40. However the large number of turns contributes to the thickness of relays; a relay's coil typically contributes more than half the thickness of the relay.
FIG. 7 is a schematic diagram a typical circuit for driving coils of a set of N relays 40. One end of each relay's coil 48 is connected to a voltage source 54 while the other end of the relay's coil is connected to ground through one of a set of N switches 49 controlled by one of control signals C1-CN. For example when a control signal C1 turns on one of switches 49, the current passes through relay coil 48 thereby causing the relay's contacts 47 to close. When control signal C1 turns off switch 49, current stops passing though coil 48 and allows contacts 47 to open.
When switch 49 opens, the magnetic field produced by coil 48 collapses producing a transient voltage spike across coil 48 that is limited by a diode 56 connected across the coil. Without diode 56 the voltage spike would pass though voltage source 54 and appear as undesirable noise in other circuits receiving power from voltage source 54. However while diode 56 reduces the amount of switching noise produced by relay 40, it also adds to the bulk of the relay.
What is needed is a low profile relay for mounting on a printed circuit board wherein the relay occupies relatively little space above the circuit board and which can be packed densely on a circuit board.